Prince William has promised to run a marathon.

Prince William

Prince William

The 34-year-old royal told 'Good Morning Britain' presenter Sean Fletcher at an event staged on behalf of the mental health organisation Heads Together that he hopes to run a marathon in Kenya at some stage in the future.

Sean shared: "I chatted to Prince William, and he has promised that he will run a marathon in Kenya sometime."

The Prince made the pledge after he learned that Sean is running the 26.2-mile course for charity Young Minds after his son was diagnosed with obsessive-compulsive disorder.

But it appears Prince William's wife, the Duchess of Cambridge, remains sceptical about whether the future King will follow through on his promise.

Sean said: "Then I spoke to Kate and she said, 'I'll believe it when I see it.'"

He said, too, that the Prince - who shares three-year-old Prince George and 20-month-old Princess Charlotte with his wife - came across as a "warm" and genuine man.

Sean explained: "This is all very new to me, I'm learning very fast because of my son's situation.

"William's understanding was incredible but also his sympathy and empathy with me.

"He asked me lots of questions. I've never met him before but he's such a warm man. Like two dads talking together."

The same sentiment was echoed by Channel 5 news presenter Sian Williams, who will be running the London Marathon after recovering from breast cancer.

She said: "The amount of passion the royals have for this cause is really impressive.

"They have researched it, they have personal experience because of the jobs they do - William taking about his role in Air Ambulance and Harry and his role in the military.

"You get a very keen sense it's something they have been aware of for a very long time and they feel as though they can help get the talking going."